OP, I definately agree that this is something I'd want to be informed of if I were a guest, but if I were DH's Cousin, I'd really appreciate it if you brought your concerns to my attention first.

Granted, DH's Cousin should absolutely have informed her prospective guests of her intentions when she invited them, (I love high dudgeon's wording) but when your Aunt was telling you about her wasabi taters, you might have said "Wow, those sound pretty tasty, but do you think MIL and FIL are going to be able to enjoy them? You know how sensitive they are to spicy stuff." That way, you've given her a polite prod to discuss with Cousin her guests' needs and opened up a possible discussion about non-spicy alternatives.

It would also make it seem a lot less...subversive (maybe? I'm not sure that's quite the word I want...it sounds too harsh) when you provided MIL with a warning and an invite. If you've let her know about your concerns, DH's Cousin would be hard pressed to view your MIL & Co ending up at your mom's dinner as anything other than legitimate concern for their comfort. (As opposed to an "Ewww, Gross!" reaction)

I've never met or talked to DH's cousin (who is male, by the way ), so I think it would be a bizarre call or email to make, especially when I have no intention of going to the holiday. Anything BIL or I could have said to aunt (who is not his mother) would have been laughed off. Personally, I don't think 2 people out of a 25 person dinner are going to make much difference, so I don't feel I've "stolen" them and they hadn't yet told the grandparents if they were officially coming or not (although they usually do).

I will be interested to hear what happens with the whole thing--I will have to ask Aunt after the holiday or get the gossip from BIL (he always seems to know what is going on).

OP - I agree with you completely. I expect a core of certain foods at Thanksgiving and if they won't be present or will be drastically altered, I would be very sad. Core foods - turkey, gravy, mashed potatoes, stuffing. It wouldn't bother me if other sides changed every year, as long as the core foods are there and in recognizable form. I get depressed watching Food Network around the holidays because they always have shows with the chefs coming up with new foods ("Instead of boring mashed potatoes, try this delicious new recipe with weird, but trendy ingredients!") Argh!